Interpretive Summary: The stem rust fungus has historically caused dramatic yield losses in cultivated wheat. Stem rust posed a major threat to wheat production in the United States in the early twentieth century with major epidemics occurring between 1900 and the 1950s. In 1999, a novel highly virulent stem rust race, reported as Ug99, was identified in Uganda that has spread to neighboring countries. The potential global dispersal of this uniquely virulent pathogen makes the development of resistant cultivars quite urgent. The wheat stem rust resistance gene Sr22 confers resistance Ug99. The gene is present on a chromosomal translocation derived from wild einkorn wheat and has not been deployed in wheat cultivars due to poor agronomic performance of lines with Sr22. DNA markers were identified that tag the Sr22 resistance gene. Then a two step process was used to develop resistant lines having smaller chromosome segments derived from the wild parent. This research led to identification of multiple resistant plants with small alien introgressions. These lines may provide a more desirable source of Sr22 that can be readily used to develop new resistant cultivars.

Technical Abstract:
The wheat stem rust resistance gene Sr22 confers resistance to Puccinia graminis race TTKSK (also known as Ug99) that developed in Africa and is an immediate threat to world wheat production. The resistance gene is present on a chromosomal translocation derived from Triticum boeoticum Boiss., which has a genome that is partially homologous to the A genome of T. aestivum L.. Sr22 has not been deployed in wheat cultivars due to poor agronomic performance of lines with the gene. Linkage analysis of SSR markers on chromosome 7AL was performed to identify loci closely linked to Sr22. A two step process was used to develop resistant lines having smaller chromosome segments derived from the diploid donor. First, individuals in which a recombination event had occurred between wheat chromosome 7A and the Sr22 introgression were identified. Increased recombination between markers was observed in 398 F3:4 lines derived from the recombinant F2 progeny. This led to identification of multiple resistant individuals with smaller alien introgressions. Resistant lines were identified having less than 6% of chromosome arm 7AL derived from T. boeoticum. These lines may provide a more desirable source of Sr22 that can be readily used to develop new resistant cultivars.